High grade astrocytomas (HGA) remain fatal without effective treatment. Using an inducible Cre-driven adult astrocyte-specific system, we explored the relative roles of key pathways perturbed in human glioblastomas (grade IV; GBM) in initiation and progression of HGA. The likely event combinations (engineered and spontaneous) yielding disease indicate grade-specific roles for each aberration and suggest specific progression mechanisms from grade II [induced only by pRb-tumor suppression (TS) inactivation] to III (addition of KrasG12D activation, with spontaneous inactivation of p53 by mutation or mislocalization), to Grade IV [further addition of PTEN inactivation (spontaneous or engineered)] without IDH1 mutation. In the transition from grade II to III disease and subsequent to KrasG12D activation, Trp53 missense mutations congruent with human GBM mutations. This study underscores the importance of stochastic events with evident tumor heterogeneity in order to recapitulate disease properties. Importantly, murine GBM transcriptomes showed concordance with the highly aggressive human mesenchymal GBM subclass. Multiple receptor tyrosine kinases (RTK) coactivation has been implicated in the etiology of glioblastoma multiforme (GBM) as well as resistance to RTK inhibitors. We found previously that disruption of RTK/Ras signaling by constitutive activation of mutant KRAS progressed the low grade to high grade astrocytoma (HGA) in vivo. Here we report that multiple RTKs were coactivated in this murine model as an intrinsic resistant mechanism. Coactivation of RTKs was not mutually exclusive with KRAS activation, suggesting RTKs and KRAS may signal to different downstream effectors. Furthermore, genetic deletion of EGFR in vivo did not show any beneficial effects on survival, which is consistent with the minimal efficacy of EGFR inhibitors in the clinic. The fact that EGFR is dispensable for HGA development may be explained by activation of more RTKs upon EGFR deletion, indicating acquired resistant mechanism. In contrast to the resistance to EGFR inhibition, murine GBMs showed high sensitivity to PI3K inhibitors. Thus, blockade of RTK downstream signaling hubs may provide the only effective targeted therapy for cancers in which multiple RTKs are active. This study provides important insight into the resistance mechanisms and has important clinical implication for molecular targeted GBM treatment. Song Y, Zhang Q, Bash R, Kutlu B, Difilippantonio S, Yin C, Gilbert D, Wang C, Yang C, Bullitt E, Kafri T, McCarthy K, Louis D, Hood L, Miller CR, Van Dyke T. An evolutionary path to glioblastoma: Insight into etiology from engineered mice. Submitted Song,Yurong1, Qian Zhang2, Rajaa Elmeskini3, Debra J. Gilbert1, Sophie Wang1, T. Norene O'Sullivan1, Teresa Sullivan1, Sandra S. Burkett4, Philip Martin3, David Threadgill5, C. Ryan Miller6, Zoe Weaver Ohler3, and Terry Van Dyke1,* Intrinsic and Acquired Resistance to EGFR Blockade: Rationale for Downstream Targeting in Glioblastoma. In preparation